The present invention relates to a sound absorbing member having a light transmissive property.
Conventionally, sound absorbing members such as a plastic foaming member formed in a fiber mat or including communication pores have been often used in the ceiling, walls, and the like for a building. These sound absorbing members are required by no means to have a light transmissive property and thus most of them do not have the light transmissive property.
In recent years, as a ceiling member or a wall member for a certain type of building having a membrane structure or as a composite material with a lighting window used in a factory, there has been demanded a sound absorbing member which has a light transmissive property. However, conventionally, there has not been developed this type of sound absorbing member. Quite lately, a sound absorbing member using a glass fiber mat has been available on the market. However, it has only such a light transmittance of 25% or less that an image cannot be seen through the member.
The sound absorbing property of a sound absorbing material is determined by the following conditions: That is, (1) the flow resistance within the material, (2) the porosity of the material, (3) the thickness of the material, (4) the background conditions, and the like. When a sound wave enters the surface of the material, then it travels through the air in gaps within the material into the interior of the material. At that time, part of the sound energy is converted to heat energy due to the viscous friction of the air to thereby absorb a sound energy. At the same time, sound absorption is also produced due to heat conduction between the air in the slight gaps of the material and the slight gap walls. Therefore, in order to obtain a certain degree of sound absorption, the density of the material must be increased and the diameter of a fiber must be decreased. As a result, even if a transparent glass fiber is used, the light transmissive property of the glass fiber is lost, which results in the low light transmittance.